LiftMaster 71-1550B72QKH Drive Sprocket — #50, 72 Teeth, 1-1/4-Inch Bore

The LiftMaster 71-1550B72QKH is a 72-tooth drive sprocket specified for #50 chain, with a 1-1/4-inch bore and a 3/8-inch keyway. The supplier also identifies two set screws. It must be selected as part of the approved operator drive ratio and shaft arrangement rather than by chain size alone. Select the replacement by the complete part number and the installed equipment configuration. A similar photograph or a shared LiftMaster brand name does not establish interchangeability. Confirm the applicable service drawing before ordering so the repair addresses the correct component and its related mounting, connection and operating requirements.

Compatibility — Models, Versions and Exclusions

Identify the Assembly and Plan the Repair

The manufacturer’s MAXUM sprocket bulletin treats sprocket selection as an application decision. A different tooth count changes the drive relationship; it is not simply an alternative diameter that may be installed wherever space permits. Have the technician verify the specified combination for the actual door and operator before changing a ratio.

Inspect chain condition, alignment, shaft engagement and the mating key. Two set screws do not eliminate the need for the correct key and approved retention procedure. A new sprocket should not be fitted over a damaged keyway or used with worn chain simply to get the door moving again.

Compare the replacement with the installed service arrangement before dismantling the equipment. Photograph the part’s location and record the complete model and visible service numbers. Inspect the surrounding attachment points and mating components for wear, impact damage or misalignment. Replacing one worn item will not correct a damaged support or another component that continues to impose abnormal loads on the repaired assembly.

Plan the repair around the manufacturer’s instructions for the actual equipment revision. Confirm access, safe support and any separate hardware before removing the old part. After installation, restore the intended retention, alignment and guarding, then test the equipment under controlled conditions. The repair is complete only when the original fault has been addressed and the system’s required operating and protection functions have been checked.

Specifications and Product Identification

Before Ordering

• Confirm all four dimensions: chain, teeth, bore and keyway.
• Check the approved operator/door sprocket combination.
• Inspect the shaft, key and mating drive.
• Confirm required guards and retention hardware.

Package Contents and Separate Requirements

The listing identifies the sprocket, with two set screws in the supplier description. Do not assume that chain, a shaft key, companion sprocket, guard or complete drive assembly is included. Confirm the current package and obtain any separate components required by the approved drive configuration.

Safety Warnings — Read Before Installation or Service

• WARNING: A moving commercial door and exposed drive sprocket — #50, 72 teeth, 1-1/4-inch bore can cause serious injury or death. Have a trained door-systems technician perform installation and service.
• Disconnect mains power and any battery backup before mechanical work or wiring. Secure the door against unexpected movement as directed by its manufacturer.
• Do not work on springs, cables or counterbalance components without appropriate training. Never bypass safety sensors or increase force to overcome binding.
• Restore guards and complete the model-specific travel, reversal and protection-device tests before returning the door to normal operation.

Related Products and Important Differences

• CPS-U monitored photo eyes — separate protection devices for approved commercial-door controls; match the operator and environment.
• TDC12X1BMC MAXUM trolley operator — a complete-operator comparison, not evidence that this legacy service part fits MAXUM.

Frequently Asked Questions

Does #50 chain compatibility prove fit?

No. Bore, keyway, hub, ratio and alignment must also agree.

What is the bore size?

The documented bore is 1-1/4 inches.

Can I select it solely by door speed?

No. Use the approved application and drive-selection information.

Is the shaft key included?

Its inclusion is not established here; confirm the package.

Need Help Confirming the Right Product?

Contact Garage-DoorParts.com before ordering. Send the full equipment model, readable part labels and photographs of the installed assembly. Include the dimensions, electrical configuration or accessory details relevant to your project so the listing can be checked against the actual installation.